Search

The natural van der Waals superlattice MnBi2Te4-(Bi2Te3)m provides an optimal platform to combine topology and magnetism in one system with minimal structural disorder. Here, we show that this system can harbor both ferromagnetic (FM) and antiferromagnetic (AFM) orders and that these magnetic orders can be controlled in two different ways by either varying the Mn-Mn distance while keeping the Bi2Te3/MnBi2Te4 ratio constant or vice versa. We achieve this by creating atomically engineered sandwich structures composed of Bi2Te3 and MnBi2Te4 layers. We show that the AFM order is exclusively determined by the Mn-Mn distance whereas the FM order depends only on the overall Bi2Te3/MnBi2Te4 ratio regardless of the distance between the MnBi2Te4 layers. Our results shed light on the origins of the AFM and FM orders and provide insights into how to manipulate magnetic orders not only for the MnBi2Te4-Bi2Te3 system but also for other magneto-topological materials., Comment: 25 pages, 5 figures, accepted to Nano Letters

The field of spintronics has seen a surge of interest in altermagnetism due to novel predictions and many possible applications. MnTe is a leading altermagnetic candidate that is of significant interest across spintronics due to its layered antiferromagnetic structure, high Neel temperature (TN ~ 310 K) and semiconducting properties. We present results on molecular beam epitaxy (MBE) grown MnTe/InP(111) films. Here, it is found that the electronic and magnetic properties are driven by the natural stoichiometry of MnTe. Electronic transport and in situ angle-resolved photoemission spectroscopy show the films are natively metallic with the Fermi level in the valence band and the band structure is in good agreement with first principles calculations for altermagnetic spin-splitting. Neutron diffraction confirms that the film is antiferromagnetic with planar anisotropy and polarized neutron reflectometry indicates weak ferromagnetism, which is linked to a slight Mn-richness that is intrinsic to the MBE grown samples. When combined with the anomalous Hall effect, this work shows that the electronic response is strongly affected by the ferromagnetic moment. Altogether, this highlights potential mechanisms for controlling altermagnetic ordering for diverse spintronic applications., Comment: Accepted in Advanced Functional Materials

Two-dimensional van der Waals magnets hosting topological magnetic textures, such as skyrmions, show promise for applications in spintronics and quantum computing. Electrical control of these topological spin textures would enable novel devices with enhanced performance and functionality. Here, using electron microscopy combined with in situ electric and magnetic biasing, we show that the skyrmion chirality, whether left-handed or right-handed, in insulating Cr2Ge2Te6, is controlled by external electric field direction applied during magnetic field cooling process. The electric-field-tuned chirality remains stable, even amid variations in magnetic and electric fields. Our theoretical investigation reveals that nonzero Dzyaloshinskii-Moriya interactions between the nearest neighbors, induced by the external electric field, change their sign upon reversing the electric field direction, thereby facilitating chirality selection. The electrical control of magnetic chirality demonstrated in this study can be extended to other non-metallic centrosymmetric skyrmion-hosting magnets, opening avenues for future device designs in topological spintronics and quantum computing.

In the recent past, MnTe has proven to be a crucial component of the intrinsic magnetic topological insulator (IMTI) family [MnTe]m[Bi2Te3]n, which hosts a wide range of magneto-topological properties depending on the choice of m and n. However, bulk crystal growth allows only a few combinations of m and n for these IMTIs due to the strict limitations of the thermodynamic growth conditions. One way to overcome this challenge is to utilize atomic layer-by-layer molecular beam epitaxy (MBE) technique, which allows arbitrary sequences of [MnTe]m and [Bi2Te3]n to be formed beyond the thermodynamic limit. For such MBE growth, finding optimal growth templates and conditions for the parent building block, MnTe, is a key requirement. Here, we report that two different hexagonal phases of MnTe-nickeline (NC) and zinc-blende/wurtzite (ZB-WZ) structures, with distinct in-plane lattice constants of 4.20 +/- 0.04 A and 4.39 +/- 0.04 A, respectively-can be selectively grown on c-plane Al2O3 substrates using different buffer layers and growth temperatures. Moreover, we provide the first comparative studies of different MnTe phases using atomic-resolution scanning transmission electron microscopy and show that ZB and WZ-like stacking sequences can easily alternate between the two. Surprisingly, In2Se3 buffer layer, despite its lattice constant (4.02 A) being closer to that of the NC phase, fosters the ZB-WZ instead, whereas Bi2Te3, sharing the same lattice constant (4.39 A) with the ZB-WZ phase, fosters the NC phase. These discoveries suggest that lattice matching is not always the most critical factor determining the preferred phase during epitaxial growth. Overall, this will deepen our understanding of epitaxial growth modes for chalcogenide materials and accelerate progress toward new IMTI phases as well as other magneto-topological applications.

Experimental manipulation of superconductivity is of paramount importance, not only for practical applications but also for identifying the key factors involved in electron pairing. In this work, we have undertaken a meticulous study of the superconductivity in a series of titanium compounds with a rocksalt structure, synthesized as epitaxial films. We find that substituting nitrogen (N) for oxygen (O) in titanium monoxide (TiO) with the stoichiometry close to TiO$_{0.6}$N$_{0.4}$ leads to superconductivity with a transition temperature (T$_c$) of ~2.6 K, about five times higher than that of TiO at ~0.5 K and half as high as the T$_c$ of ~6 K in titanium nitride (TiN). However, Eliashberg theoretical calculations predict similar Tc in TiO, Ti oxynitride and TiN. The analysis of electron mean free path suggests the presence of significant disorder in TiO and a remarkable reduction in the impact of disorder in oxynitrides. Density functional theory (DFT) calculations reveal that disorder decreases the coherence of electronic states for non-zero momenta, which would degrade the influence of electron-phonon. Our findings demonstrate the disorder and superconductivity depend strongly on the N/O ratio, highlighting the critical role of disorder for superconductivity in this series of Ti(O,N) materials.

Van der Waals (vdW) magnetic materials such as Cr2Ge2Te6 (CGT) show promise for novel memory and logic applications. This is due to their broadly tunable magnetic properties and the presence of topological magnetic features such as skyrmionic bubbles. A systematic study of thickness and oxidation effects on magnetic domain structures is important for designing devices and vdW heterostructures for practical applications. Here, we investigate thickness effects on magnetic properties, magnetic domains, and bubbles in oxidation-controlled CGT crystals. We find that CGT exposed to ambient conditions for 5 days forms an oxide layer approximately 5 nm thick. This oxidation leads to a significant increase in the oxidation state of the Cr ions, indicating a change in local magnetic properties. This is supported by real space magnetic texture imaging through Lorentz transmission electron microscopy. By comparing the thickness dependent saturation field of oxidized and pristine crystals, we find that oxidation leads to a non-magnetic surface layer which is thicker than the oxide layer alone. We also find that the stripe domain width and skyrmionic bubble size are strongly affected by the crystal thickness in pristine crystals. These findings underscore the impact of thickness and surface oxidation on the properties of CGT such as saturation field and domain/skyrmionic bubble size and suggest a pathway for manipulating magnetic properties through a controlled oxidation process., Comment: 11 pages, 6 figures

In Mott materials strong electron correlation yields a spectrum of complex electronic structures. Recent synthesis advancements open realistic opportunities for harnessing Mott physics to design transformative devices. However, a major bottleneck in realizing such devices remains the lack of control over the electron correlation strength. This stems from the complexity of the electronic structure, which often veils the basic mechanisms underlying the correlation strength. Here, we present control of the correlation strength by tuning the degree of orbital overlap using picometer-scale lattice engineering. We illustrate how bandwidth control and concurrent symmetry breaking can govern the electronic structure of a correlated $SrVO_3$ model system. We show how tensile and compressive biaxial strain oppositely affect the $SrVO_3$ in-plane and out-of-plane orbital occupancy, resulting in the partial alleviation of the orbital degeneracy. We derive and explain the spectral weight redistribution under strain and illustrate how high tensile strain drives the system towards a Mott insulating state. Implementation of such concepts will drive correlated electron phenomena closer towards new solid state devices and circuits. These findings therefore pave the way for understanding and controlling electron correlation in a broad range of functional materials, driving this powerful resource for novel electronics closer towards practical realization.

The layer stacking order in 2D materials strongly affects functional properties and holds promise for next generation electronic devices. In bulk, octahedral MoTe$_2$ possesses two stacking arrangements, the Weyl semimetal T$_d$ phase, and the higher-order topological insulator 1T' phase; however, it remains unclear if thin exfoliated flakes of MoTe$_2$ follow the T$_d$, 1T', or an alternative stacking sequence. Here, we resolve this debate using atomic-resolution imaging within the transmission electron microscope. We find that the layer stacking in thin flakes of MoTe$_2$ is highly disordered and pseudo-random, which we attribute to intrinsic confinement effects. Conversely, WTe$_2$, which is isostructural and isoelectronic to MoTe$_2$, displays ordered stacking even for thin exfoliated flakes. Our results are important for understanding the quantum properties of MoTe$_2$ devices, and suggest that thickness may be used to alter the layer stacking in other 2D materials., Comment: 6 figures, 2 tables

Epitaxial Fe(Te,Se) thin films have been grown on various substrates but never been realized on magnetic layers. Here we report the epitaxial growth of four-fold Fe(Te,Se) film on a six-fold antiferromagnetic insulator, MnTe. The Fe(Te,Se)/MnTe heterostructure shows a clear superconducting transition at around 11 K and the critical magnetic field measurement suggests the origin of the superconductivity to be bulk-like. Structural characterizations suggest that the uniaxial lattice match between Fe(Te,Se) and MnTe allows a hybrid symmetry epitaxy mode, which was recently discovered between Fe(Te,Se) and Bi2Te3. Furthermore, Te/Fe flux ratio during deposition of the Fe(Te,Se) layer is found to be critical for its superconductivity. Now that superconducting Fe(Te,Se) can be grown on two related hexagonal platforms, Bi2Te3 and MnTe, this result opens a new possibility of combining topological superconductivity of Fe(Te,Se) with the rich physics in the intrinsic magnetic topological materials (MnTe)n(Bi2Te3)m family., Comment: 19 pages, 5 figures, accepted by Nano Letters

The manifestation of skyrmions in the Mott-insulator Cu$_2$OSeO$_3$ originates from a delicate balance between magnetic and electronic energy scales. As a result of these intertwined couplings, the two symmetry-inequivalent magnetic ions, Cu-I and Cu-II, bond into a spin S=1 entangled tetrahedron. However, conceptualizing the unconventional properties of this material and the energy of the competing interactions is a challenging task due the complexity of this system. Here we combine X-ray Absorption Spectroscopy and Resonant Inelastic X-ray Scattering to uncover the electronic and magnetic excitations of Cu$_2$OSeO$_3$ with site-specificity. We quantify the energies of the 3d crystal-field splitting for both Cu-I and Cu-II, fundamental to optimize model Hamiltonians. Additionally, we unveil a site-specific magnetic mode, indicating that individual spin character is preserved within the entangled-tetrahedron picture. Our results thus provide experimental constraint for validating theories that describe the interactions of Cu$_2$OSeO$_3$, highlighting the site-selective capabilities of resonant spectroscopies

The inverse spinel ferrimagnetic NiCo$_2$O$_4$ possesses high magnetic Curie temperature $T_C$, high spin polarization, and strain-tunable magnetic anisotropy. Understanding the thickness scaling limit of these intriguing magnetic properties in NiCo$_2$O$_4$ thin films is critical for their implementation in nanoscale spintronic applications. In this work, we report the unconventional magnetotransport properties of epitaxial (001) NiCo$_2$O$_4$ films on MgAl$_2$O$_4$ substrates in the ultrathin limit. Anomalous Hall effect measurements reveal strong perpendicular magnetic anisotropy for films down to 1.5 unit cell (1.2 nm), while $T_C$ for 3 unit cell and thicker films remains above 300 K. The sign change in the anomalous Hall conductivity ($\sigma_{xy}$) and its scaling relation with the longitudinal conductivity ($\sigma_{xx}$) can be attributed to the competing effects between impurity scattering and band intrinsic Berry curvature, with the latter vanishing upon the thickness driven metal-insulator transition. Our study reveals the critical role of film thickness in tuning the relative strength of charge correlation, Berry phase effect, spin orbit interaction, and impurity scattering, providing important material information for designing scalable epitaxial magnetic tunnel junctions and sensing devices using NiCo$_2$O$_4$., Comment: 12 pages, 4 figures

Intrinsic, two-dimensional (2D) ferromagnetic semiconductors are an important class of materials for spin-charge conversion applications. Cr$_2$Ge$_2$Te$_6$ retains long-range magnetic order in bilayer at cryogenic temperatures and shows complex magnetic interactions with considerable magnetic anisotropy. Here, we performed a series of structural, magnetic, X-ray scattering, electronic, thermal transport and first-principles calculation studies which reveal that localized electronic charge carriers in Cr$_2$Ge$_2$Te$_6$ are dressed by surrounding lattice and are involved in polaronic transport via hopping that is sensitive on details of magnetocrystalline anisotropy. This opens possibility for manipulation of charge transport in Cr$_2$Ge$_2$Te$_6$ - based devices by electron-phonon- and spin-orbit coupling-based tailoring of polaron properties.

It is challenging to grow an epitaxial four-fold compound superconductor (SC) on six-fold topological insulator (TI) platform due to stringent lattice-matching requirement. Here, we demonstrate that Fe(Te,Se) can grow epitaxially on a TI (Bi2Te3) layer due to accidental, uniaxial lattice match, which is dubbed as "hybrid symmetry epitaxy". This new growth mode is critical to stabilizing robust superconductivity with TC as high as 13 K. Furthermore, the superconductivity in this FeTe1-xSex/Bi2Te3 system survives in Te-rich phase with Se content as low as x = 0.03 but vanishes at Se content above x = 0.56, exhibiting a phase diagram that is quite different from that of the conventional Fe(Te,Se) systems. This unique heterostructure platform that can be formed in both TI-on-SC and SC-on-TI sequences opens a route to unprecedented topological heterostructures., Comment: 41 pages, 14 figures, accepted by Nano Letters

Controlling magnetic order in magnetic topological insulators (MTIs) is a key to developing spintronic applications with MTIs, and is commonly achieved by changing the magnetic doping concentration, which inevitably affects spin-orbit-coupling strength and the very topological properties. Here, we demonstrate tunable magnetic properties in topological heterostructures over a wide range, from a ferromagnetic phase with Curie temperature of around 100 K all the way to a paramagnetic phase, while keeping the overall chemical composition the same, by controlling the thickness of non-magnetic spacer layers between two atomically-thin magnetic layers. This work showcases that spacer-layer control is a powerful tool to manipulate magneto-topological functionalities in MTI heterostructures. Furthermore, the interaction between the MTI and the Cr2O3 buffer layers also led to robust topological Hall effect surviving up to a record-high 6 T of magnetic field, shedding light on the critical role of interfacial layers in thin film topological materials., Comment: 25 pages, 5 figures, accepted by Nano Letters

The origin of strain-induced ferromagnetism, which is robust regardless of the type and degree of strain in LaCoO3 (LCO) thin films, is enigmatic despite intensive research efforts over the past decade. Here, by combining scanning transmission electron microscopy with ab initio density functional theory plus U calculations, we report that the ferromagnetism does not emerge directly from the strain itself, but rather from the creation of compressed structural units within ferroelastically formed twin-wall domains. The compressed structural units are magnetically active with the rocksalt-type high-spin/low-spin order. Our study highlights that the ferroelastic nature of ferromagnetic structural units is important for understanding the intriguing ferromagnetic properties in LCO thin films.

In the quest to understand high-temperature superconductivity in copper oxides, a vigorous debate has been focused on the pseudogap - a partial gap that opens over portions of the Fermi surface in the 'normal' state above the bulk critical temperature ($T_{c}$). The pseudogap has been attributed to precursor superconductivity, to the existence of preformed pairs, or to competing orders such as charge-density waves. A direct determination of the charge of carriers as a function of temperature and bias could help resolve among these alternatives. Here, we report measurements of the shot noise of tunneling current in high-quality La$_{2-x}$Sr$_{x}$CuO$_{4}$/La$_{2}$CuO$_{4}$/La$_{2-x}$Sr$_{x}$CuO$_{4}$ (LSCO/LCO/LSCO) heterostructures fabricated using atomic-layer-by-layer molecular beam epitaxy, for several doping levels. The data delineate three distinct regions in the bias voltage-temperature ($V-T$) space. Well outside the superconducting gap region, the shot noise agrees quantitatively with independent tunneling of charge-e carriers. Deep within the gap, shot noise is greatly enhanced, reminiscent of multiple Andreev reflections. Starting above $T_{c}$ and extending to biases much larger than the gap, there is a broad region in which the noise substantially exceeds the expectations of single-charge tunneling, indicating pairing of carriers. Pairs are detectable deep into the pseudogap region of temperature and bias., Comment: Author's version of an article published in Nature on 21 August 2019. Link to the published paper: https://www.nature.com/articles/s41586-019-1486-7

Ferroelectric materials are characterized by degenerate ground states with multiple polarization directions. In a ferroelectric capacitor this should manifest as equally favourable up and down polarization states. However, this ideal behavior is rarely observed in ferroelectric thin films and superlattice devices, which generally exhibit a built-in bias which favors one polarization state over the other. Often this polarization asymmetry can be attributed to the electrodes. In this study we examine bias in PbTiO$_3$-based ferroelectric superlattices that is not due to the electrodes, but rather to the nature of the defects that form at the interfaces during growth. Using a combination of experiments and first-principles simulations, we are able to explain the sign of the observed built-in bias and its evolution with composition. Our insights allow us to design devices with zero built-in bias by controlling the composition and periodicity of the superlattices.

Topological crystalline insulator tin telluride (SnTe) provides a rich playground to examine interactions of correlated electronic states, such as ferroelectricity, topological surface states, and superconductivity. Making SnTe into nanowires further induces novel electronic states due to one-dimensional (1D) confinement effects. Thus, for transport measurements, SnTe nanowires must be made narrow in their diameters to ensure the 1D confinement and phase coherence of the topological surface electrons. This study reports a facile growth method to produce narrow SnTe nanowires with a high yield using alloy nanoparticles as growth catalysts. The average diameter of the SnTe nanowires grown using the alloy nanoparticles is 85 nm, nearly a factor of three reduction from the previous average diameter of 240 nm using gold nanoparticles as growth catalysts. Transport measurements reveal the effect of the nanowire diameter on the residual resistance ratio and magnetoresistance. Particularly, the ferroelectric transition temperature for SnTe is observed to change systematically with the nanowire diameter. In situ cryogenic cooling of narrow SnTe nanowires in a transmission electron microscope directly reveals the cubic to rhombohedral structural transition, which is associated with the ferroelectric transition. Thus, these narrow SnTe nanowires represent a model system to study electronic states arising from the 1D confinement, such as 1D topological superconductivity as well as a potential multi-band superconductivity., Comment: 5 figures

We present first room-temperature thermoelectric signature of the skyrmion lattice. This was observed in Fe3Sn2, a Kagome Dirac crystal with massive Dirac fermions that features high-temperature skyrmion phase. The room-temperature skyrmion lattice shows magnetic-field dependence of the wavevector whereas thermopower is dominated by the electronic diffusion mechanism, allowing for the skyrmionic bubble lattice detection. Our results pave the way for the future skyrmion-based devices based on the manipulation of the thermal gradient.

Van der Waals (vdW) semiconductors are attractive for highly scaled devices and heterogeneous integration since they can be isolated into self-passivated, two-dimensional (2D) layers that enable superior electrostatic control. These attributes have led to numerous demonstrations of field-effect devices ranging from transistors to triodes. By exploiting the controlled, substitutional doping schemes in covalently-bonded, three-dimensional (3D) semiconductors and the passivated surfaces of 2D semiconductors, one can construct devices that can exceed performance metrics of 'all-2D' vdW heterojunctions. Here, we demonstrate, 2D/3D semiconductor heterojunctions using MoS2 as the prototypical 2D semiconductor laid upon Si and GaN as the 3D semiconductor layers. By tuning the Fermi levels in MoS2, we demonstrate devices that concurrently exhibit over seven orders of magnitude modulation in rectification ratios and conductance. Our results further suggest that the interface quality does not necessarily affect Fermi-level tuning at the junction opening up possibilities for novel 2D/3D heterojunction device architectures., Comment: Manuscript plus supporting information, 4 Figures

Two-dimensional (2D) van der Waals (vdW) materials show a range of profound physical properties that can be tailored through their incorporation in heterostructures and manipulated with external forces. The recent discovery of long-range ferromagnetic order down to atomic layers provides an additional degree of freedom in engineering 2D materials and their heterostructure devices for spintronics, valleytronics and magnetic tunnel junction switches. Here, using direct imaging by cryo-Lorentz transmission electron microscopy we show that topologically nontrivial magnetic-spin states, skyrmionic bubbles, can be realized in exfoliated insulating 2D vdW Cr2Ge2Te6. Due to the competition between dipolar interactions and uniaxial magnetic anisotropy, hexagonally-packed nanoscale bubble lattices emerge by field cooling with magnetic field applied along the out-of-plane direction. Despite a range of topological spin textures in stripe domains arising due to pair formation and annihilation of Bloch lines, bubble lattices with single chirality are prevalent. Our observation of topologically-nontrivial homochiral skyrmionic bubbles in exfoliated vdW materials provides a new avenue for novel quantum states in atomically-thin insulators for magneto-electronic and quantum devices.

Quantum anomalous Hall effect(QAHE) can only be realized at extremely low temperatures in magnetically doped topological insulators(TIs) due to limitations inherent with the doping precess. In an effort to boost the quantization temperature of QAHE, magnetic proximity effect in magnetic insulator/TI heterostructures has been extensively investigated. However, the observed anomalous Hall resistance has never been more than several Ohms, presumably owing to the interfacial disorders caused by the structural and chemical mismatch. Here, we show that, by growing (BixSb1-x)2Te3(BST) thin films on structurally and chemically well-matched, ferromagnetic-insulating CeGeTe3(CGT) substrates, the proximity-induced anomalous Hall resistance can be enhanced by more than an order of magnitude. This sheds light on the importance of structural and chemical match for magnetic insulator/TI proximity systems., Comment: 33 pages, 10 figures including supporting information,accepted by Nano Letters

A key feature of the topological surface state under a magnetic field is the presence of the zeroth Landau level at the zero energy. Nonetheless, it has been challenging to probe the zeroth Landau level due to large electron-hole puddles smearing its energy landscape. Here, by developing ultra-low-carrier density topological insulator Sb$_2$Te$_3$ films, we were able to reach an extreme quantum limit of the topological surface state and uncover a hidden phase at the zeroth Landau level. First, we discovered an unexpected quantum-Hall-to-insulator-transition near the zeroth Landau level. Then, through a detailed scaling analysis, we found that this quantum-Hall-to-insulator-transition belongs to a new universality class, implying that the insulating phase discovered here has a fundamentally different origin from those in non-topological systems., Comment: 22 pages, 5 figures, Appendix (28 pages)

The search for new topological materials and states of matter is presently at the forefront of quantum materials research. One powerful approach to novel topological phases beyond the thermodynamic space is to combine different topological/functional materials into a single materials platform in the form of superlattices. However, despite some previous efforts, there has been a significant gap between theories and experiments in this direction. Here, we provide the first detailed set of experimentally-verifiable phase diagrams of topological superlattices composed of archetypal topological insulator (TI), Bi$_{2}$Se$_{3}$, and normal insulator (NI), In$_{2}$Se$_{3}$, by combining molecular-level materials control, low-temperature magnetotransport measurements, and field theoretical calculations. We show how the electronic properties of topological superlattices evolve with unit-layer thicknesses and utilize the weak antilocalization effect as a tool to gain quantitative insights into the evolution of conducting channels within each set of heterostructures. This orchestrated study opens the door to the possibility of creating a variety of artificial-topological-phases by combining topological materials with various other functional building blocks such as superconductors and magnetic materials.

The morphology and dimension of the conductive filament formed in a memristive device are strongly influenced by the thickness of its switching medium layer. Aggressive scaling of this active layer thickness is critical towards reducing the operating current, voltage and energy consumption in filamentary type memristors. Previously, the thickness of this filament layer has been limited to above a few nanometers due to processing constraints, making it challenging to further suppress the on-state current and the switching voltage. Here, we study the formation of conductive filaments in a material medium with sub-nanometer thickness, formed through the oxidation of atomically-thin two-dimensional boron nitride. The resulting memristive device exhibits sub-nanometer filamentary switching with sub-pA operation current and femtojoule per bit energy consumption. Furthermore, by confining the filament to the atomic scale, we observe current switching characteristics that are distinct from that in thicker medium due to the profoundly different atomic kinetics. The filament morphology in such an aggressively scaled memristive device is also theoretically explored. These ultra-low energy devices are promising for realizing femtojoule and sub-femtojoule electronic computation, which can be attractive for applications in a wide range of electronics systems that desire ultra-low power operation., Comment: 19 pages, 4 figures, Advanced Materials, in press

Topological insulators (TI) are a phase of matter that host unusual metallic states on their surfaces. Unlike the states that exist on the surface of conventional materials, these so-called topological surfaces states (TSS) are protected against disorder-related localization effects by time reversal symmetry through strong spin-orbit coupling. By combining transport measurements, angle-resolved photo-emission spectroscopy and scanning tunneling microscopy, we show that there exists a critical level of disorder beyond which the TI Bi2Se3 loses its ability to protect the metallic TSS and transitions to a fully insulating state. The absence of the metallic surface channels dictates that there is a change in topological character, implying that disorder can lead to a topological phase transition even without breaking the time reversal symmetry. This observation challenges the conventional notion of topologically-protected surface states, and will provoke new studies as to the fundamental nature of topological phase of matter in the presence of disorder., Comment: Accepted in PRB

In typical topological insulator (TI) systems the TI is bordered by a non-TI insulator, and the surrounding conventional insulators, including vacuum, are not generally treated as part of the TI system. Here, we implement the first material system where the roles are reversed, and the TSS form around the non-TI (instead of the TI) layers. This is realized by growing a layer of the tunable non-TI $(Bi_{1-x}In_{x})_{2}Se_{3}$ in between two layers of the TI $Bi_2Se_3$ using the atomically-precise molecular beam epitaxy technique. On this tunable inverse topological platform, we systematically vary the thickness and the composition of the $(Bi_{1-x}In_{x})_{2}Se_{3}$ layer and show that this tunes the coupling between the TI layers from strongly-coupled metallic to weakly-coupled, and finally to a fully-decoupled insulating regime. This system can be used to probe the fundamental nature of coupling in TI materials and provides a tunable insulating layer for TI devices., Comment: Accepted in PRB

Material defects remain as the main bottleneck to the progress of topological insulators (TIs). In particular, efforts to achieve thin TI samples with dominant surface transport have always led to increased defects and degraded mobilities, thus making it difficult to probe the quantum regime of the topological surface states. Here, by utilizing a novel buffer layer scheme composed of an In2Se3/(Bi0.5In0.5)2Se3 heterostructure, we introduce a quantum generation of Bi2Se3 films with an order of magnitude enhanced mobilities than before. This scheme has led to the first observation of the quantum Hall effect in Bi2Se3., Comment: 33 pages, to be published in Nano Letters

The recent emergence of topological insulators (TI) has spurred intensive efforts to grow TI thin films on various substrates. However, little is known about how robust the topological surface states (TSS) are against disorders and other detrimental effects originating from the substrates. Here, we report observation of a well-defined TSS on Bi2Se3 films grown on amorphous SiO2 (a-SiO2) substrates and a large gating effect on these films using the underneath doped-Si substrate as the back gate. The films on a-SiO2 were composed of c-axis ordered but random in-plane domains. However, despite the in-plane randomness induced by the amorphous substrate, the transport properties of these films were superior to those of similar films grown on single-crystalline Si(111) substrates, which are structurally better matched but chemically reactive with the films. This work sheds light on the importance of chemical compatibility, compared to lattice matching, for the growth of TI thin films, and also demonstrates that the technologically-important and gatable a-SiO2/Si substrate is a promising platform for TI films., Comment: Accepted for publication in Applied Physics Letters

Multiferroic hexagonal RMnO3 (R=rare earths) crystals exhibit dense networks of vortex lines at which six domain walls merge. While the domain walls can be readily moved with an applied electric field, the vortex cores were so far impossible to control. Our experiments demonstrate that shear strain induces a Magnus-type force pulling vortices and antivortices in opposite directions and unfolding them into a topological stripe domain state. We discuss the analogy between this effect and the current-driven dynamics of vortices in superconductors and superfluids.

The ferroelectric (FE) control of electronic transport is one of the emerging technologies in oxide heterostructures. Many previous studies in FE tunnel junctions (FTJs) exploited solely the differences in the electrostatic potential across the FTJs that are induced by changes in the FE polarization direction. Here, we show that in practice the junction current ratios between the two polarization states can be further enhanced by the electrostatic modification in the correlated electron oxide electrodes, and that FTJs with nanometer thin layers can effectively produce a considerably large electroresistance ratio at room temperature. To understand these surprising results, we employed an additional control parameter, which is related to the crossing of electronic and magnetic phase boundaries of the correlated electron oxide. The FE-induced phase modulation at the heterointerface ultimately results in an enhanced electroresistance effect. Our study highlights that the strong coupling between degrees of freedom across heterointerfaces could yield versatile and novel applications in oxide electronics., Comment: 23 pages, 5 figures, 1 supporting information

Field-induced switching of ferroelectric domains with a topological vortex configuration is studied by atomic imaging and electrical biasing in an electron microscope, revealing the role of topological defects on the topologically-guided change of domain-wall pairs in a hexagonal manganite.